Voltage to current converters are useful for a variety of applications, including variable gain amplifiers and voltage controlled oscillators.
A first conventional voltage to current converter comprises two stacked bipolar diodes and a unity-gain connected feedback amplifier with a bipolar pnp output transistor. Linearly varying the anode-to-cathode voltage V.sub.d of such a bipolar diode stack results in an output current which varies approximately exponentially. However, the voltage-current output characteristic of such a voltage to current converter is highly susceptible to fabrication process variations. Additionally, the first conventional voltage to current converter requires external regulation of input voltage to avoid an input overdrive.
A second conventional voltage to current converter comprises a common emitter bipolar transistor pair. The bases have an input voltage applied, but the emitters are biased with a constant current. The second conventional voltage to current converter has a well-defined voltage-current characteristic. That is, the collector current is strictly a function of input voltage and bias current. However, it provides a hyperbolic tangent-type output characteristic which only approximates a desired substantially exponential output characteristic. Furthermore, the hyperbolic tangent-type output characteristic approximates an exponential output characteristic for only a limited range of input voltage levels.